A hypothesis is presented to explain cell recognition, selective cell adhesion, cell sorting out in embryonic cells, the invasiveness of normal tissue by tumor cells and the general problem of metastasis of cancer cells. The hypothesis states that cells sort out by non-random movement leading to a segregation of cells according to species and/or cell types. The sorting out is accompanied by the formation of microvilli in specific response to neighboring cells and the production of a specific extracellular material to bind similar cells together. Using echinoderm embyronic cells the following studies are proposed to test the hypothesis: to isolate and further characterize the extracellular material and to study its effect on the reaggregation process, to study sorting out of embryonic cell types, to establish the non-random movement of cells within an aggregate, to carry out a scannig electron microscope study of reaggregation, to determine the role of paternal gene(s) in the reaggregation process, and to study the effects of cytochalasin B and colchicine on reaggregation. BIBLIOGRAPHIC REFERENCE: Spiegel, M. and E. S. Spiegel. 1975. The Reaggregation of Dissociated Embryonic Sea Urchin Cells. Amer. Zool. 15:583-606.